A typical air bag inflator includes a rupturable burst disk which, when ruptured, releases a flow of gas from a container in which gas is stored to an air bag to inflate the air bag. The burst disk ruptures in response to a vehicle collision. In known inflators which include a rupturable burst disk, the opening created by the ruptured burst disk is a factor in determining the rate of gas flow into the air bag. It is important, therefore, to have the burnt disk reliably rupture to create an opening with an effective flow area in a predetermined range of areas.